1. Field of the Invention
This invention relates to downhole safety valves for controlling the flow of fluids from wells. More particularly, the invention relates to safety valves which may be controlled from the surface and which also automatically close in response to a situation which would result in an oil spill or possibly a fire in the event of damage to equipment above the safety valve.
2. Description of the Prior Art
Oil and gas wells, particularly those located off-shore, are susceptible to wellhead damage caused, for example, by violent storms, fires, collision with a vessel, or other accident or disaster. To prevent uncontrolled flow of fluids from the well through the damaged wellhead, it is common practice to install a downhole safety valve in the tubing through which the fluids are produced. Such safety valves are designed to be normally open so that well fluids may be produced therethrough. In response to some signal from the surface or to a change in flow conditions through the tubing, the valve can be closed to stop flow of fluid through the tubing. One popular type of safety valve used for this purpose utilizes a hydraulic line extending from the safety valve to the surface of the well. Hydraulic pressure is maintained in the line to hold the safety valve open, and in the event of accident the pressure in the control line is released and the safety valve automatically closes. These prior art subsurface safety valves controlled by hydraulic pressure have been widely used in the field, and generally operate satisfactorily. However, hydraulically controlled, subsurface, wireline or tubing retrievable tubing safety valves do have significant disadvantages. For example, if the control tubing leading to the subsurface valve is damaged, corroded, or otherwise leaks to permit reduction of the hydraulic control pressure, the safety valve will close in accordance with its "fail-safe design" and the well will be shut in. To restore production, the tubing must be pulled and the hydraulic control line replaced. If the wireline retrievable valve requires service due to failure, the valve must be retrieved, and this action permits pressure communication between the production tubing and the oil well casing annulus. This is hazardous in the case of high pressure wells as it allows high well pressure to be imposed on the casing annulus. A pack-off mandrel to seal the hydraulic port in the downhole safety valve could be run. Such a pack-off mandrel would in itself be an obstruction in the tubing which would have to be removed whenever it was necessary to conduct wireline operations below the safety valve. Upon removal of the pack-off mandrel, pressure communication between the production tubing and the casing annulus would necessarily result.
Another disadvantage to the use of hydraulically controlled, wireline retrievable, subsurface tubing safety valves is that extremely high hydraulic control pressures are sometimes required. This means that hydraulic control lines at the surface must carry high hydraulic pressure exceeding 10,000 psi in some cases. On offshore platforms particularly, these high hydraulic control pressures constitute a potential hazard to personnel working on the platform and around the wellhead.
Another widely used type of safety valve operates in response to excessive flow rates to automatically close the production tubing. This type of device has the disadvantage that it does not shut off the well in the event of damage to equipment above the safety valve unless the well is flowing at a rate sufficient to actuate the shutoff mechanism.
There has been a long standing need for a subsurface tubing safety valve which is reliable, which automatically closes in the event of damage to equipment regardless of the rate of flow through the tubing, and which is not subject to the disadvantage of allowing high tubing pressure to be communicated to the casing annulus. Such a device is provided by the present invention.